Filter system for multiple channel amplifiers



April 9, 1940.

v. D. LANDON ET AL 2,196,266

FILTER SYSTEM FOR MULTIPLE CHANNEL AMPLIFIERS Filed Nov. 28, 1936 5 22 M a 45 a /7 P/C7Y/A& 2L J27 gal/N0, T0, 1 Tag P/CTMPE 3 mug {fa/m VEANOA/ amwoa/v FAEQU'A/C) By 3116721454 0110 fl/VA maentor attorney Patented Apr. 9, 1940 UNITED] STATES FILTER. SYSTEM FOR MULTIPLE CHANNEL ANIPLIFIERS Vernon D.'Landon, Haddonfield, and Gustave L. Grundmann, Westmont, N. J., assignors to Radio Corporation of America, a corporation of I Delaware 7 Application November 28, 1936, Serial No. 113,158

5 Claims.

The present invention relates to a filter .sys-

term for amultiple channel amplifier wherein signals in a relatively wide band of frequencies, and signals in a relatively narrow band or at a single frequency may be separated, and separately amplified, and has for its object to provide an improved filter for separating, the-signals at differing frequencies.

The invention also has for its object to provide a filter of the above type which may be utilized advantageously between or as coupling means for high impedance high-gain amplifier tubes of the screen-grid type.

The system of the present invention is particularly adapted to television picture and sound amplifiers for separating the picture signal from the sound signal, although it may be utilized to separate signals in any two differing adjacent frequency bands.

It is, therefore, afurther object of the present invention to provide in a multiple signal channel amplifier a sharply selective band-pass filter of the T-network type comprising an M-derived section to which a single input circuit may be connected and from which a pair of output circuits may be coupled to high impedance input tubes forming a part'of a multiple channel amplifier.

A filter of the type to which the invention relates, having M-derived sections, is described in U. S. Patent 1,538,964, issued May 26, 1925, to O. J. Zobel.

The invention willibe further understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out inthe appended claims.

In the drawing, I

Figure 1 is a schematic circuit diagram of a filter system embodying the invention and applied to a multiple channel signal transmission circuit,

Figure 2 is a further schematic circuit diagram showing the application of'the'circuit of- Fig. 1 to an amplifying channel embodying high gainhigh impedance amplifier tubes, and

Figure 3 is a curve showing a feature of the operation of the circuit of Figs. 1 and 2. 7

Referring to Fig. 1, by way of illustration, a television picture and sound amplifying channel is represented by the leads-5,5 and is connected through a filter network 'I with separate output circuits comprising leads 8-!) and 9I0.

The filter shown is of the band pass type having a T-network form in which series in- .ductances II and I2 are connected between the leads 5 andfi and ashunt inductance I3 and series capacity I 4 therefor are connected as the shunt branch of the filter. This filter is provided with a compensating resistor i5 connected between an input terminal I6 and an output terminal II at opposite ends of the series connected inductances Ii and I2, whereby the shunt branch I3iil is permitted to introduce a high degree of attenuation at its resonance frequency. This method of compensation is described in the U. S. patent to Bode, 2,002,216.

In a band pass signal circuit, the network is tuned to provide a band pass characteristic of the type indicated by the curve I 8 in Fig. 3, having a relatively wide flat top characteristic as indicated, and shunt circuit or stem of-the T- network is tuned to an adjacent channel frequency such as indicated at the point I9 thereby required. Therefore, in accordance with the invention, the inductances II and I2 are made relatively high in value sufficiently to be tuned by the shunt tube capacities provided in an ampli-' fier. For example, the capacity '26 may be the plate capacity of a preceding amplifier, the plate impedance of which is indicated by the series resistor 22 in the input circuit and the capacity 2I may be provided by the grid capacity of the output amplifier tube to which the network is connected through the leads 8 and 9.

In a similar manner, the inductance I3 maybe relatively high in value whereby it is tunable to the cut-off frequency by the grid capacity of the amplifier to which the output leads 9 and I0 may be connected and representedin the circuit of Fig. 1 by the capacity I4.

The connections in an amplifier by which the foregoing results are obtained are indicatedin Fig. 2, to which attention is now directed and in which like parts throughout, as used in, Fig. bear the same reference numerals.

Referring to Fig. 2, a portion of a combined sound and picture signal intermediate frequency amplifier circuit is shown. This comprises an in- I put lead 23 for the sound and picture signals,

connected to a screen-grid amplifier tube 25, the

anode output circuit 25 of which is coupled through a coupling impedance 26 and coupling capacitor 21 to the attenuating and separating network I. The network is of the T-type having series inductances II and I2 with a shunt circuit including the inductance I3.

The shunt capacity for the input, corresponding to the capacity I4 of Fig. 1, is provided by the grid-cathode capacity of a screen grid amplifier tube 28 coupled through impedance coupling means with a terminal 29 on the series resonant circuit which includes the inductance l3. Through the capacity coupling 30, the grid to cathode capacity of the tube 23 is applied between the terminal 29 and the low potential or ground side of the circuit as indicated in Fig. 1, whereby a series resonance circuit is provided without additional capacity means in series with the inductance l3. Likewise the input and output capacities 20 and 2| are provided by the plate capacity of the tube 24 and the grid capacity of a tube 3!, the latter being connected with the output lead 8. These capacities are indicated in dotted lines in connection with the circuit dia gram of Fig. 2 and may readily be referred to the capacities shown in Fig. l.

The inductances II and I2 are tuned to desired resonance to provide the band pass character istic shown in Fig. 3 by means of adjustable magnetite cores indicated in association with each i the inductances, and the resistor i5 is adjusted to provide compensation for the resistance of the shunt circuit through the inductance I3 and the capacity M.

With this arrangement as shown in Fig. 3, an 8.75 megacycle sound signal may be derived from the network while at the same time the filter serves to remove the sound signal from the picture signal which is passed through the filter in a band substantially 1.5 megacycles wide.

It has been found that a relatively high voltage at the rejection, or in this case the sound frequency, exists at the terminal 29 or at a terminal point 32 in the circuit of Fig. 1 whereby the sound channel of the amplifier may be connected therewith to receive a relatively strong signal without the intermediary of initial amplifiers. Thus the T-network filter may be utilized to separate signals in a relatively wide band from a single adjacent signal frequency without additional circuit elements other than a simple filter of the type shown.

It is desirable and substantially essential, in connection with high gain-high impedance tubes of the screen grid type, to use relatively high inductances. In this way. existing internal tube capacities may provide fixed tuning capacities in conjunction with the adjustable inductances. Thus the tube capacities and relatively high inductance values may be combined to advantage in producing sharp cut-off in the filter and effective separation of the sound from the picture signal.

We claim as our invention:

1. In a television amplifier, means providing a picture and sound amplifying channel including high gain electric discharge amplifier devices, a band pass filter comprising a T-network interposed between certain of said amplifier devices. including a variable inductance device and means providing an output tap connection for one of said amplifier devices in the stem of the T-network, said variable inductance and the tube input capacity forming series resonant elements in said stem and said output connection being interposed between said elements.

2. In an intermediate frequency amplifier for signals in a relatively wide frequency band and signals at an adjacent channel frequency, the combination of a band pass filter for said relatively wide band signals including two series in ductance arms, means including internal tube capacities in the input and output circuits of said filter providing shunt tuning capacities for said series arms, means including inductance and capacity elements providing a series resonant circuit as a shunt arm of the filter network responsive to said adjacent channel frequency, and amplifier means for deriving said adjacent channel signals from said shunt arm, said last named means having an input connection across the capacity element of said shunt arm.

3. In a band pass amplifier, the combination of a band pass filter comprising a T-network forming an interstage coupling means, whereby the filter is tuned to a pass band in part by the associated tube capacities, and a series resonant rejector circuit for signals outside the pass band comprising tube capacity and an inductance element for attenuating said last named signals in the band pass circuits of the filter, and means for deriving said signals from the rejector circuit including a tube providing said last named capacity.

4. In a multiple signal channel amplifier, an M-derived filter network comprising series inductance elements and a shunt inductance element connected between said series inductance elements to provide a T-network, amplifier tubes coupled in cascade through said T-network, said inductance elements having such relatively high inductance and being adjustable whereby they are tunable to resonance by the internal tube capacities of said amplifier tubes, said series inductances and said tube capacities being resonant in approximately the center of a relativcl. wide pass band, and said shunt inductance having capacity in series therewith with which it resonates at a frequency adjacent to said pass band, means for deriving from said network through said series inductance elements signals in said predetermined wide band, and means providing an output amplifier connection for deriving therefrom amplified signals at and near the frequency of resonance of the shunt inductance and capacity in series therewith.

5. In a multiple signal channel amplifier, an M-derived filter network comprising series in ductance elements and a shunt inductance element connected between said series inductance elements to provide a T-network, said inductance elements being adjustable, amplifier tubes coupled in cascade through said T-network, means including the internal tube capacities in said amplifier tubes and additional shunting capacity for tuning said inductances to resonance. said series inductance elements and said capacities being resonant approximately at the center of a relatively wide pass band and said shunt inductance having capacity in series therewith with which it resonates at a frequency adjacent to said pass band, means for deriving from said network through said series inductance elements signals in said predetermined wide band, and means providing an output amplifier connection across the tuning capacity means in circuit with the shunt inductance element for deriving therefrom amplified signals at and near the frequency o resonance of the shunt inductance and the capacity in series therewith.

VERNON D. LANDON. GUSTAVE L. GRUNDMANN. 

